Try this add-on to minimize Firefox to the "system tray" then set your Windows 7 preferences for the system tray:
*Add-on - https://addons.mozilla.org/en-US/firefox/addon/minimizetotray-revived/
**I use the above add-on; you can also search at - https://addons.mozilla.org/en-US/firefox/
*Windows 7 system tray - http://www.makeuseof.com/tag/manage-and-tidy-the-windows-7-system-tray/